PROJECT SUMMARY Cannabis is currently the most used drug in the US, particularly in young people. It is also associated with the highest demand for intervention; however, current treatment success rates are low, with 70% of motivated treatment-seekers relapsing early, during the withdrawal phase (< 3 weeks from last use). Cannabis withdrawal, a clinically recognized but poorly understood syndrome, is a risk factor for relapse and a target for intervention. There are currently no approved pharmacotherapies for cannabis use disorder (CUD), while a severe deficit exists in understanding the neurobiology of cannabis withdrawal syndrome. Preclinical and human genetic studies have implicated Fatty Acid Amide Hydrolase (FAAH), the enzyme metabolizing the major endocannabinoid anandamide in cannabis withdrawal, suggesting that higher metabolic activity (i.e.: lower anandamide) may be associated with greater withdrawal symptomatology and that increasing endocannabinoids may decrease the severity of cannabis withdrawal. In our NIH-funded pilot study we found that levels of this enzyme are low in CUD after overnight cannabis cessation (12 hours) and relate to levels of cannabis metabolites. Thus, we propose that low levels of FAAH during early abstinence act to delay the appearance of withdrawal and that as levels increase, symptomatology manifests. Our data indicate the need for scanning subjects during acute withdrawal in order to understand how FAAH contributes to cannabis withdrawal and, by extension, to relapse. We are uniquely suited to investigate the role of FAAH in CUD and cannabis withdrawal as we have collected preliminary data in this condition; we have the clinical and scientific expertise in the field of CUD and imaging and have the only available neuroimaging tool to investigate this question. There is an urgent need to accelerate the translation, into clinic, of pharmacotherapies already in development, by providing neuroimaging information which could advance our understanding of endocannabinoid metabolism in CUD. Following up on our finding of decreased FAAH activity in CUD during early abstinence, our major specific aim is to establish, by PET imaging, whether FAAH levels increase during acute withdrawal (from overnight to 72 hours) and are associated with clinical symptoms. The potential impact of this project is important, as it provides the first imaging investigation of endocannabinoid metabolism during cannabis withdrawal. The knowledge generated by this project will provide the basis for the further development of evidence-based therapeutic approaches targeting FAAH.